Piezoelectric materials exhibit an electro-mechanical coupling, allowing them to develop an electric charge proportional to an applied mechanical stress in a direct mode. Alternatively, in a converse mode, they can generate a strain from the application of an external electric field. This phenomenon, first discovered in 1880 by J. and P. Curie, is essential to modern devices such as parking sensors, medical ultrasound, fuel injection valves and the ubiquitous piezoelectric buzzer. These and other typical applications span a plethora of industries, but can be categorised as transducers having the following modes of operation; (i) effector, actuator and motor mode for use in speakers, fuel/ink injection, robotics and MEMs; (ii) transducer mode employed in ultrasonic imaging in both medical and SONAR platforms, active vibration dampening, electronic frequency filters and finally; (iii) sensor mode for pickups, microphones and gas lighters. In fact the market space is covered by over 100,000 application patents and valued at >$10 bn annually, and is dominated by sales of transducers used in effector and transducer mode made from lead zirconate titanate (PZT).
PZT contains a high proportion of lead (Pb), an element which is banned in all other electronic applications under the European Union directive Restriction of Hazardous Substances (RoHS). Currently electro-ceramics are exempt, as lead plays a vital role in providing the polarisation required in these materials, but the exemption is often reviewed, and any change would require suppliers and customers to find lead free alternatives.
Principally however, although PZT is effectively exploited in a wide range of markets it is fundamentally limited to applications below ˜200° C., above which the stability of its piezoelectric properties decrease steadily.
High temperature electronics is an area of research that has offered materials and design challenges since the 1970's, and has been recognised as a field of significant industrial importance. Currently environments of repeated thermal cycles of 150° C. are now typical, with the requirement for operation at up to 500° C. becoming increasingly necessary in applications such as electronics, including sensors for deep oil drilling, automotive actuator mode transducers for increased operating efficiency, nuclear and other ‘clean’ energy solutions with extreme environments. The aerospace industry too now requires sensors and effector mode transducers to withstand temperatures exceeding 500° C. for over 100,000 hours in the air and in space to increase efficiency, conduct health monitoring and reduce mass. For example an increase in the operating temperature of a gas turbine by 150° C. removes the necessity for turbine cooling components and can subsequently raise the thermal efficiency by 6%.
The temperature limitations of PZT and other conventional piezoelectric materials can be partially overcome in several ways; however each of these solutions have clear downsides. For example, one can distance the transducer from the high temperature environment by using some intermediary material. However, the sensitivity and bandwidth of the transducer is negatively impacted. Alternatively, one can prevent overheating by either inserting the transducer into the high temperature environment intermittently, or cooling the transducer with a liquid such as water. However, complicated engineering solutions may be required to effectively prevent overheating in this manner. A further possibility is to simply accept that the materials will have a short lifespan in the high temperature environment, but regularly replacing the transducers can be both costly and time consuming. Similar problems exist when employing materials in environments having high radiation or pressure.
Extensive effort has already been made in the development of high temperature transducers employing fibre optics, precious metal strain gauges and piezoelectrics, particularly aimed at surface (SAW) and bulk acoustic wave (BAW) transducers for pressure, mass and chemical measurements. Piezoelectric transducers have been proven to offer excellent resolution, temperature stability, sensitivity and low cost integration properties compared to the other devices for measuring charge, voltage and frequency dependent mechanisms.
The materials used for these applications range in compositions, forms and structures, but importantly aim to have as high a piezoelectric operation temperature above the mainstay PZT system as possible. These include single crystals such as quartz, lithium niobate and gallium orthophosphate, thin films like aluminium nitride and polycrystalline materials, such as bismuth titanate, which are most likely to be used in industrial applications due to their low cost of processing and ease of integration into common electrical devices. These are dominated by bismuth based ceramics as well as including mixed phase systems akin to the mechanism in PZT which provides its premium piezoelectric properties.
Recently, a new piezoelectric ceramic material has been developed which can withstand high temperatures. The material, referred to hereinafter as BF-KBT-PT, is described in International Patent Publication WO2012/013956 A1, the entirety of which is hereby incorporated by reference. Although BF-KBT-PT is able to withstand higher temperatures, conventional piezoelectric transducers cannot be simply modified to include the new material because other components used in the transducers—e.g. the backing material, the casing, the wiring and solder and other elements—are not equipped to withstand the required conditions.
It is an aim of the present disclosure to address at least some of the above difficulties, or other difficulties which will be appreciated from the description below.